This post is a follow-up to an already closed topic JACK buffer size is not remembered when Ardour closed started by @martin.vlk.
If you use PipeWire with JACK, the issue is that Ardour can only request a different buffer size for the duration of a session. Once you restart Ardour, the size will default back to whatever PipeWire has configured, because JACK applications automatically use the buffer-size chosen by the server.
A simple workaround is to use the PIPEWIRE_LATENCY environment variable which accepts a buffer size and sample rate.
Simply set up the values before starting Ardour, e.g.: PIPEWIRE_LATENCY=64/48000 ardour7.
I tested this on my Arch Linux and it works as expected. Hope this helps.